he's a marketable name. he is a 'stable enough' QB. he's an insanely talented athlete, he's just not bright or all that focused. he is good enough for 12-14 years of keeping it together. And on any given sunday, he *could* put it together and win. He pushed manning hard last year in shootouts. 75% of the QBs in the league can't shoot it out like him, maybe more.
he's a safe choice for QB. May never win the big one, may never get there, but might get lucky one year. Favre only won it once, and he was much better than Romo, it was every year losing to 49ers or Cowboys.